One type of device that converts one DC voltage level to another is commonly known as a DC-to-DC converter (or “DC-DC” converter). DC-DC converters are commonly included in battery-operated devices such as mobile telephones, laptop computers, etc., in which the various subsystems of the device require several discrete voltage levels. In some types of devices, such as a mobile telephone that operates in a number of different modes, it is especially desirable to supply certain elements, such as power amplifiers, with a supply voltage at the most efficient level for the mode of operation, rather than waste power and accordingly drain the battery prematurely. In such devices, it is desirable to employ a DC-DC converter that can generate a greater number of discrete voltage levels.
Several types of DC-DC converters are known, including switched-mode DC-DC converters and DC-DC converters that employ pulse-width modulation (PWM). Switched-mode DC-DC converters convert one DC voltage level to another by storing the input energy momentarily in inductors or capacitors and then releasing that energy to the output at a different voltage. The switching circuitry thus continuously switches between two states or phases: a first state in which a network of inductors or capacitors is charging, and a second state in which the network is discharging. The switching circuitry can be configured to generate an output voltage that is a fixed fraction of the battery voltage, such as one-third, one-half, two-thirds, etc., where a mode selection signal is provided as an input to the switching circuitry to control which of the fractions is to be employed. Different configurations of the network of inductors or capacitors can be selected by manipulating switches in the network using the mode selection signal.
The number of discrete output voltages that a switched-mode DC-DC converter can generate is related to the number of inductors or capacitors. In a portable, handheld device such as a mobile telephone it is desirable to minimize size and weight. A DC-DC converter having a large number of inductors or capacitors is not conducive to minimizing the size and weight of a mobile telephone. A PWM-based DC-DC converter can generate a larger number of discrete voltages than a switched-mode DC-DC converter without employing significantly more inductors, capacitors or other elements. However, a PWM-based DC-DC converter can generate a large spectrum of spurious output signals that can adversely affect the operation of a mobile telephone or other frequency-sensitive device. Filters having large capacitances or inductances can be included in a PWM-based DC-DC converter to minimize these spurious signals, but large filter capacitors or inductors are undesirable for the same reasons described above.